This invention relates to an adhesive composition for an optical disk. More particularly, this invention relates to an adhesive composition that will be useful for producing an optical disk that is used as an optical recording medium having a structure in which transparent substrates are bonded to each other and which is provided with at least one reflecting metal film, and to an optical disk using the adhesive composition.
Optical disks known as a high-density recording medium such as DVD have been used vigorously as a recording medium in versatile fields for recording music and images, and for computer-associated recording. In such optical disks, digital signals recorded on the disk are detected as the changes of reflection intensity of a semiconductor laser beam emitted from an optical read head so as to read recorded data. These disks include read-only disks, disks capable of writing but not capable of erasing, and disks capable of both writing and erasing.
When two substrates are bonded to each other by bonding, bonding by using a photocuring type adhesive is effective for a one-side type disk (in which recorded data is read only from one side) because rays of light transmit from the side of a transparent substrate (a dummy substrate), and this bonding method has been mainly employed for a practical production of DVD. In a double-side type (in which recorded data are read from both sides), on the other hand, the photocuring type adhesive cannot be used because opaque substrates are bonded to each other. Thermosetting adhesives (an epoxy type, a melamine type, a urethane type, an acrylic type, etc.) and hot-melt adhesives have been used in the past in order to bond such opaque substrates to each other.
However, the bonding method using the thermosetting adhesive or a hot-melt adhesive requires heating, and involves the problem of warp and deformation of the substrates. The disk bonded by the hot-melt adhesive has the problem of low thermal stability. Therefore, a delay curing photo-cationic setting type epoxy adhesive has been widely used to bond opaque substrates to each other.
When the delay curing photo-cationic setting type epoxy adhesive is used, the adhesive is first applied to the substrates before bonding, and the rays of light are irradiated to start the reaction. The substrates are bonded during this reaction and are left standing until the adhesive is completely set. This method involves the problem that adjustment of the reaction time is difficult. If the reaction time is designed excessively short, the reaction proceeds excessively before bonding so that bonding is likely to be insufficient. When the reaction time is excessively long, on the other hand, the time for keeping the substrates fixed gets elongated so as to prevent deviation and deformation of the substrates until adhesion is completed after bonding of the substrates. Productivity is remarkably deteriorated in this case. In the adhesive of this type, the catalyst creates an acid during the reaction. Though this acid is necessary for the reaction, the acid corrodes the recording film. Fillers for entrapping the acid are therefore added to prevent the corrosion. In consequence, the adhesive becomes likely to separate and to become opaque, and appearance of the inner peripheral transparent portion of the disk after bonding (a region not having signal recording) is deteriorated. This adhesive is applied by a screen-printing system. At this time, fine bubbles are entrapped in the applied adhesive and remain after bonding, too. Though this problem is not much severe in the past, it is believed to become critical when the recording capacity of the optical disk increases in future.
An attempt has been made also to use a two-part adhesive for bonding transparent substrates for DVD. Japanese Unexamined Patent Publication (Kokai) No. 7-37274 mixes a two-part adhesive by using a mixing applicator and then applies the mixture. In this case, since the setting reaction starts occurring immediately after mixing, the time from the application to bonding is limited. If a blend design that retards the setting reaction is employed, the problem of productivity occurs in the same way as in the case of the photo-cationic setting type epoxy adhesive. The adhesive used in this Japanese Unexamined Patent Publication (Kokai) No. 7-37274 uses an acrylic acid ester monomer having a hydroxyl group in its molecule. However, this reference does not disclose an adhesive that uses this adhesive in combination with urethane acrylate having polytetramethylene glycol as a skeleton.
Japanese Examined Patent Publication (Kokoku) No. 4-43332 discloses a production method of an optical disk using a two-part type acrylic adhesive, wherein a first part is applied to one of disk substrates and a second part, to the other substrate, for bonding them together. However, the adhesive used is known (commercially available) and is not developed specifically in consideration of durability of optical disks. Japanese Unexamined Patent Publication (Kokai) Nos. 11-120630 and 11-126379 describe the use of two-part type adhesives, but these adhesives are not specifically developed for optical disks, either.
Utilization of optical disks such as DVD as high-density recording media has been expanded, but no adhesive has yet been obtained that facilitates the production of double-side type read system disks that are more suitable for high-density recording, by mutually bonding substrates each provided with a signal recording layer.
It is, therefore, an object of the present invention to provide a novel adhesive composition that makes it easier to mutually bond substrates each having a signal recording layer without deteriorating characteristics of a disk produced, such as durability.
According to one aspect of the present invention, there is provided a two-part adhesive composition comprising first and second parts each containing urethane acrylate having a polytetramethylene glycol skeleton and an acrylate having a hydroxyl group in its molecule, wherein the first part further contains diacyl peroxide and the second part further contains a tertiary amine.
Either one, or both, of the first and second parts may further contain a photo-polymerization initiator.
According to another aspect of the present invention, there is provided an optical disk including disk substrates bonded to each other and at least one reflecting metal film, wherein the disk substrates are bonded by using a two-part adhesive composition according to the present invention.
Urethane acrylate described above is preferably an oligomer comprising at least portions originating from tetramethylxylene diisocyanate, hydroxyethyl acrylate and ethylene glycol, respectively, besides the skeletal portion of polytetramethylene glycol.
The acrylate having a hydroxyl group in its molecule described above is preferably at least one kind of the members selected from the group consisting of hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate and 2-hydroxy-3-phenoxypropyl acrylate, and more preferred are the former three members.